Method for the production of a semiconductor device

ABSTRACT

A method for making a semiconductor device having a pattern of highly doped regions located some distance apart in a semiconductor substrate and regions of low doping located between the highly doped regions. A diffusion barrier material is applied to the semiconductor substrate at the location of the regions of low doping by imprinting with the barrier material in the pattern of the regions of low doping. The doping material is applied after or before imprinting with barrier material so that the highly doped regions are formed essentially between the barrier material in the substrate.

BACKGROUND OF THE INVENTION

The invention relates to a method for making a semiconductor devicehaving a pattern of highly doped regions located some distance apart ina semiconductor substrate and regions of low doping located between thehighly doped regions, wherein

a doping material is applied to the substrate, at least in the locationof the highly doped regions,

the substrate is subjected to a diffusion step in which atoms diffusefrom the doping material into the substrate, and

conducting contacts are made above the highly doped regions.

A method for making a selective emitter in a p-type crystalline Sisubstrate, with which a diffusion material in the form of a dopingpaste, such as phosphorus paste, is applied to the substrate by screenprinting is described in J. Horzel, J. Szlufeik, J. Nijs and R. Mertens,“A simple processing sequence for selective emitters”, 26^(th) PVSC,September 30-October 3; Anaheim, Calif.; 1997 IEEE pp 139-142. Thesubstrate is then dried on a conveyor belt and placed in a diffusionfurnace. During the diffusion step the doping materials diffuse into thesubstrate while diffusion material moves to the regions outside theimprint of doping material via the gas atmosphere in the furnace.Relatively deep diffusion zones having a phosphorus concentrationvarying from 10²⁰ at the surface of the substrate to 10¹⁷ at a depth of0.5 μm below the substrate surface are formed below the imprinted dopematerial. Shallow diffusion zones having a low phosphorus concentration,varying from 10¹⁹ at the substrate surface to 10¹⁸ at a depth of 0.2 μm,are formed outside the region of the imprint.

The disadvantage of the known method, in particular in the case of theproduction of solar cells in which the highly doped regions are arrangedin a pattern of a series of parallel tracks or fingers, is that thediffusion between the tracks having a high concentration is highlysensitive to the atmosphere in the diffusion furnace, as a result ofwhich the diffusion method is insufficiently stable as a productionprocess. Furthermore the ratio between the high and low doping isdependent and therefore local doping cannot be adjusted to the optimum.To obtain good contact with the metalization placed on the highly dopedregions, which metalization is frequently applied by screen printing, alow surface resistance, and thus as high as possible doping, is desired.For the regions located between the metalization an increase in yield ispossible, for example in the case of n-p-type solar cells, bypassivation of the surface with thermal SiO₂ or PECVD SiN, as a resultof which recombination of charge carriers at the surface iscounteracted. This increase in yield can be achieved only if the dopingis low.

SUMMARY OF THE INVENTION

One aim of the present invention is therefore to provide a method formaking a semiconductor device, in particular a solar cell, with whichregions of high and low doping can be applied efficiently in accuratelydetermined positions on the substrate. A further aim of the invention isto provide a method with which the concentrations of the doping materialin the regions of high and low doping can be adjusted relativelyindependently of one another.

To this end the method according to the invention is characterized inthat before the diffusion step a diffusion barrier material is appliedto the substrate at the location of the regions of low doping byimprinting with the barrier material in the pattern of the regions oflow doping.

During the diffusion step, which usually will be carried out attemperatures of approximately 900° C., the substrate regions locatedbeneath the barrier material are shielded by the latter from thediffusion material applied to the neighboring regions. As a result theconcentration in the regions of low doping can be freely adjustedaccurately and independently of the concentration in the highly dopedregions. Furthermore, with the method according to the invention asingle screen printing step and a single drying step can suffice.

It is possible first to apply the doping material to the substrate as auniform layer, for example by spraying, and then to print the barriermaterial by means of a printing technique onto the regions of thesubstrate with low doping, after which the diffusion step is carriedout. In this embodiment the barrier material can delay the diffusion ofthe underlying diffusion material or it can have etching properties, sothat the underlying diffusion during the diffusion step is etched out ofthe substrate. A barrier material which has etching properties is, forexample, ZnO.

Alternatively, according to the invention the barrier material is firstapplied by screen printing, stencil printing, offset printing or tamponprinting or using other printing techniques known per se to thoseregions of the substrate which are to have low doping. The dopingmaterial can then be applied as a single layer by spraying, spinning,immersing, vapor deposition or from the gas phase (such as, for example,by means of POCl₃ gas in a crystal tube) on top of the substrate and ontop of the barrier material.

Although this is not to be preferred from the production standpoint, thedoping material can also be printed selectively onto the regions of thesubstrate for high doping, before or after applying the barriermaterial. The barrier material is, for example, a dielectric materialsuch as Si₃N₄, SiO₂ or TiO₂, to which an n-type doping material, such asphosphorus (P), arsenic (As), antimony (Sb) or bismuth (Bi) can havebeen added, or a p-type doping material such as boron (B), aluminum(Al), gallium (Ga), indium (In) or thallium (Th). This material isprinted onto the substrate in paste form and then sintered attemperatures between 200° C. and 1000° C.

Following the diffusion step the surface resistance in the highly dopedregions is for example, between 10 and 60 ohm square, for aconcentration of doping atoms of between 10¹⁸ cm⁻³ and 10²¹ cm⁻³, for adiffusion depth beneath the substrate surface of between 0.1 μm and 0.5μm. The surface resistance of the regions with low doping is between 40ohm and 600 ohm square, for a concentration of doping atoms of between10¹⁷ cm⁻³ and 10²¹ cm⁻³, for diffusion depth of between 0.1 μm and 0.5μm.

BRIEF DESCRIPTION OF THE DRAWINGS

A few embodiments of the method according to the present invention willbe explained in more detail by way of example with reference to theappended diagrammatic drawing. In the drawing:

FIG. 1 shows a diagrammatic representation of a method according to theprior art,

FIGS. 2a, 2 b and 2 c show a first embodiment of a method according tothe present invention using a uniform layer of doping material,

FIGS. 3a, 3 b and 3 c show an alternative embodiment of a methodaccording to the invention with selective application of the dopingmaterial,

FIGS. 4a, 4 b and 4 c show an embodiment of the method according to theinvention where the barrier material has etching properties and

FIG. 5 shows a concentration profile of a semiconductor device producedaccording to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a p-type substrate consisting of, for example, silicondoped with n-type atoms. A doping material in the form of a paste, suchas a phosphorus paste, is applied by means of screen printing to thesubstrate 1 above those regions of the substrate 1 which are to havehigh doping. Following a diffusion step at approximately 900° C. in adiffusion furnace there are highly doped regions 3 and regions 4, 4′ oflow doping, formed by lateral diffusion from the phosphorus paste 2 viathe atmosphere in the diffusion furnace, in substrate 1.

FIG. 2a shows a first step of the method according to the invention, inwhich a barrier material 5, 5′, 5″ is applied by means of a printingtechnique, such as, for example, screen printing, to the p-typecrystalline silicon substrate 1 above those regions of the substrate 1which are to have low doping. The barrier material 5-5″ comprises, forexample, a dielectric material such as Si₃N₄, SiO₂ or TiO₂ in pasteform. After imprinting the paste the barrier material 5-5″ is sinteredat a temperature between 200° C. and 1000° C. The doping material 2 isthen applied uniformly over the substrate 1 and over the barriermaterial 5-5″, as shown in FIG. 2b. The doping material can be appliedin very many different ways, for example in the form of an organicmolecule (for example triethyl phosphate) or in the form of phosphoricacid. The doping material 2 can be applied by means of spraying,spinning, immersion, vapor deposition or from a gas phase.

The semiconductor device according to FIG. 2b is then placed in adiffusion furnace and subjected to a diffusion step at, for example,approximately 1000° C. As a result of this the n-type atoms diffuse fromthe doping material 2 into the substrate 1, so that highly doped regions6, 6′, which are located between regions 7, 7′, 7″ of low doping, areformed in the substrate 1. The regions 7, 7′, 7″ of low doping arelocated beneath the barrier material 5-5″. Finally, conducting contacts8, 8′, for example consisting of aluminum, are applied, likewise bymeans of a printing technique, to the doping material 2 on top of thehighly doped regions 6, 6′. However, it is also possible to etch awaythe doping material 2 and the barrier material 5-5″ after the diffusionstep in FIG. 2b and then to apply a passivating layer consisting of, forexample, SiO₂ or PECVD SiN over the substrate 1.

FIG. 3a shows an embodiment with which the barrier material 5-5″ isfirst of all printed on the substrate 1 in the desired pattern ofregions of low doping and highly doped regions, after which the n-typedoping material is applied between the barrier material 5-5″.

After carrying out a diffusion step in FIG. 3b, the metal contacts areapplied at 8, 8′ to the doping material 2 above the highly doped regions6, 6′ by a printing technique.

It is possible to add an etching agent to the barrier material 5-5″ inthe embodiments according to FIG. 2a-FIG. 3c in order to etch away anydoping material that has diffused beneath the barrier material.

FIG. 4a shows an embodiment with which the doping material 2 is firstapplied over the substrate 1, after which the barrier material 5-5″ isdeposited in the desired pattern onto the doping material 2 byimprinting. In this case the barrier material can comprise an etchingagent such as, for example, ZnO. During the diffusion step, which iscarried out in FIG. 4b, the etching agent from the barrier material willetch away the diffusion regions located beneath this, so that the highlydoped regions 6, 6′ remains in the substrate in positions where thebarrier material 5-5″ is absent. Metal contacts 8, 8′ can then beapplied above the highly doped diffusion regions 6, 6′, as shown in FIG.4c.

This method has the advantage that an optical difference which can beused when aligning the metalization pattern is produced between thepositions of the barrier material and neighboring locations.Furthermore, reduced reflection can be obtained with the constructionaccording to FIG. 4c.

It is pointed out that although the method has been described withreference to a p-type substrate and an n-type doping material the methodis also suitable for use with n-type substrates with p-type dopingmaterial.

Finally, FIG. 5 shows a plot of the concentration against the depthbelow the substrate surface for a semiconductor device produced inaccordance with the present invention. The process conditions for theproduction of the semiconductor device having the concentration profileaccording to FIG. 5 were as follows:

The barrier layer was applied from a print paste which was sintered inair at approximately 400° C. This leads to a layer of approximately 1 μmthick SiO₂ of low porosity (<80% volume of SiO₂). It is important thatthe paste shows few cracks in order to achieve a maximum gain inefficiency. Partial coverage of the wafer with a barrier layer leads toa lower efficiency but not to short-circuiting of the cell, as is thecase when a selective emitter is made with the aid of a resist toprotect the locations where a highly doped emitter is needed.

After applying the barrier layer, a phosphorus-containing layer wasapplied by spin coating using a phosphorus source in the liquid phase.Diffusion into the wafer was then carried at 900° C. for 10 minutes,which led to the pattern below the barrier layer as is indicated in FIG.5.

To make cells, silver lines with a width of approximately 100 μm arethen printed within the area previously etched by the barrier layer. Thesize of this etched area has been chosen to be relatively large toprevent the risk of short-circuiting with the regions of low doping.This etched area is at least 150 μm wide. It can be seen from FIG. 5that the concentration of donor atoms in the highly doped regions 6, 6′is appreciably higher and extends over a greater depth than theconcentrations of doping material in regions below the barrier material5-5″. The low donor concentrations at the surface, as are shown in FIG.5, are outstandingly suitable for surface passivation. This can lead toa significant rise in efficiency of the order of 5%, relative.

What is claimed is:
 1. A method of making a semiconductor device havinga substrate with a pattern of highly doped regions and lightly dopedregions between the highly doped regions, the method comprising thesteps of: applying a substantially continuous layer of doping materialto the substrate; creating the highly doped regions and the lightlydoped regions in the substrate by diffusing dopant atoms from the dopingmaterial into the substrate; providing conducting contacts above thehighly doped regions; and before the diffusing step, imprinting adiffusion barrier material on the substrate substantially exclusively inthe regions that are to be the lightly doped regions.
 2. The method ofclaim 1, wherein the imprinting step is before the step of applying thelayer of doping material.
 3. The method of claim 1, wherein theimprinting step is after the step of applying the layer of dopingmaterial.
 4. The method of claim 1, wherein the diffusion barriermaterial is a dielectric material in paste form and, after theimprinting step, further comprising the step of sintering the dielectricmaterial.
 5. The method of claim 1, further comprising the step ofadding a dopant to the diffusion barrier material.
 6. The method ofclaim 1, further comprising the steps of adding an etchant to thediffusion barrier material, and etching the substrate adjacent to thediffusion barrier material.
 7. The method of claim 1, wherein the highlydoped regions have a surface resistance of 10 to 60 ohms per square andthe regions between the highly doped regions have a surface resistanceof 30 to 500 ohms per square.
 8. The method of claim 7, wherein thehighly doped regions having a dopant concentration of 10¹⁸ cm⁻³ to 10²¹cm⁻³ and a diffusion depth of 0.1 μm to 0.5 μm, and wherein the regionsbetween the highly doped regions having a dopant concentration of 10¹⁷cm⁻³ to 10²¹ cm⁻³ and a diffusion depth of 0.1 μm to 0.5 μm.
 9. Themethod of claim 1, wherein the diffusing step is carried out atapproximately 900° C.
 10. A method of making a semiconductor devicehaving a substrate with a pattern of highly doped regions and lightlydoped regions between the highly doped regions, the method comprisingthe steps of: imprinting a diffusion barrier material on the substratesubstantially exclusively in the regions that are to be the lightlydoped regions, the diffusion barrier material being a dielectricmaterial in paste form; sintering the diffusion barrier material;applying a substantially continuous layer of doping material to thesubstrate and the sintered diffusion barrier material; creating thehighly doped regions and the lightly doped regions by diffusing dopantatoms from the doping material into the substrate; and providingconducting contacts on the layer of doping material above the highlydoped regions.
 11. The method of claim 10, wherein the sintering step iscarried out at 200° to 1000° C.
 12. The method of claim 10, wherein thediffusing step is carried out at approximately 1000° C.
 13. A method ofmaking a semiconductor device having a substrate with a pattern ofhighly doped regions and lightly doped regions between the highly dopedregions, the method comprising the steps of: applying a substantiallycontinuous layer of doping material to the substrate; imprinting adiffusion barrier material on the layer of doping material substantiallyexclusively in the regions that are to be the lightly doped regions, thediffusion barrier material being a dielectric material in paste formhaving an etching agent therein; creating the highly doped regions andthe lightly doped regions by diffusing dopant atoms from the dopingmaterial into the substrate and, during this diffusing step, etching thesubstrate in the lightly doped regions; and providing conductingcontacts on the layer of doping material above the highly doped regions.